Hot-rolled steel sheet exhibiting excellent cold formability and excellent surface hardness after forming

ABSTRACT

Disclosed is a hot-rolled steel sheet that has a thickness of 3 to 20 mm and includes, in a chemical composition in mass percent, C of 0.3% or less (excluding 0%), Si of 0.5% or less (excluding 0%), Mn of 0.2% to 1%, P of 0.05% or less (excluding 0%), S of 0.05% or less (excluding 0%), Al of 0.01% to 0.1%, and N of 0.008% to 0.025%, with the remainder consisting of iron and inevitable impurities. A solute nitrogen content is 0.007% or more, and the carbon and nitrogen contents meet a condition as specified by 10C+N≦3.0. The microstructure of the steel sheet includes pearlite of less than 20% in area percentage, with the remainder approximately consisting of ferrite. The average grain size of the ferrite is 3 to 35 μm. The steel sheet has good cold formability during forming and still has predetermined surface hardness after forming.

TECHNICAL FIELD

The present invention relates to a hot-rolled steel sheet that has goodcold formability during forming and still has predetermined surfacehardness after forming.

BACKGROUND ART

Recently, better fuel efficiency is required in automobiles from theviewpoint of environmental protection. To meet the requirement, steelsfor use in automobile parts such as gears and other gearbox unit partsand casings more and more require lighter weights, i.e., higherstrength. To meet the requirement of lighter weights and higherstrength, hot-forged steels prepared from steel bars by hot forging havebeen generally used. Instead of the hot forged gears and other parts,demands are increasingly made to provide these parts by cold forging soas to reduce CO₂ emission in parts production processes.

Advantageously, the cold forming (cold forging) offers higherproductivity and provides both good dimensional accuracy and good steelyield as compared with hot forming and warm forming. Disadvantageously,however, the cold forming, when employed to produce parts, has toessentially use steels having high strength, i.e., high deformationresistance so as to allow the cold-worked parts to surely have strengthat predetermined levels or higher. Unfortunately, steels with increasingdeformation resistance may more readily invite shorter lives ofcold-forming tools and more readily cause fracture/cracking upon coldforming.

To prevent this, some conventional techniques produce high-strengthparts surely having predetermined strength (hardness) by cold-forging asteel into a predetermined shape, and subjecting the cold-forged steelto a heat treatment such as quenching and tempering. However, the partsinevitably change their dimensions in the heat treatment after coldforging and thereby require secondary correction by machining such ascutting. Under these circumstances, demands have been made to provide asolution that can omit the heat treatment and the subsequent forming.

As a possible solution to the problems, for example, Patent Literature(PTL) 1 discloses that a wire rod/steel bar for cold forging havingexcellent strain aging hardening properties is obtained by preparing alow-carbon steel, restraining the progress of natural aging of the steelusing solute carbon, and allowing the steel to surely undergo agehardening by strain aging hardening at a predetermined level.

This technique, however, controls the strain aging hardening by thesolute carbon content alone and hardly gives a steel that has bothsufficient cold formability and required level of hardness/strengthafter forming.

Under the circumstances, the present applicant made variousinvestigations while focusing on the effect of solute carbon and solutenitrogen in a steel on deformation resistance and static strain aginghardening. As a result, the present applicant found that appropriatecontrol of the amounts of these solute elements gives amechanical-structure-use steel that exhibits good cold formabilityduring forming and still has predetermined surface hardness (strength)after cold forming (cold forging). The present applicant has alreadyfiled a patent application based on these findings (see PTL 2).

The steel achieves both good cold formability and higher hardness(higher strength) after forming. Disadvantageously, however, the steelis a hot-forged steel as with the wire rod/steel bar disclosed in PTL 1and suffers from high production cost. To achieve still lower productioncost, attempts have been made to produce automobile parts by coldforming using hot-rolled steel sheets instead of the conventionalhot-forged steels.

Typically, PTL 3 proposes a technique, according to which a hot-rolledsteel sheet for nitriding can have high surface hardness and asufficient hardening depth after nitriding.

Disadvantageously, however, the technique requires nitriding as an extraprocess after cold forming and fails to offer sufficiently low cost.

PTL 4 proposes a hot-rolled steel sheet that has a chemical compositioncontaining C in a content of 0.10% or less, Si in a content less than0.01%, Mn in a content of 1.5% or less, Al in a content of 020% or less,Ti and Nb in a content as specified by (Ti+Nb)/2 of 0.05% to 0.50%, Sina content of 0.005% or less, N in a content of 0.005% or less, O in acontent of 0.004% or less, so that the total content of S, N, and O be0.0100% or less. The hot-rolled steel sheet has a microstructurecontaining 95% or more of ferrite as approximately a ferritesingle-phase. The literature mentions that the hot-rolled steel sheethas excellent dimensional accuracy in a finely blanked surface, hasextremely high surface hardness of the blanked surface after forming,and still offers excellent resistance to red-scale defects.

The hot-rolled steel sheet, however, is designed to treat nitrogen as aharmful element and to control the nitrogen content to an extremely lowcontent and absolutely differs in technical idea from the hot-rolledsteel sheet according to the present invention in which nitrogen ispositively utilized.

CITATION LIST Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication (JP-A) No.Hei10(1998)-306345

PTL 2: JP-A No. 2009-228125

PTL 3: JP-A No. 2007-162138

PTL 4: JP-A No. 2004-137607

SUMMARY OF INVENTION Technical Problem

The present invention has been made while focusing these circumstances,and it is an object of the present invention to provide a hot-rolledsteel sheet that has good cold formability during forming and still haspredetermined surface hardness after forming.

Solution to Problem

The present invention provides, in one aspect, a hot-rolled steel sheethaving excellent cold formability and satisfactory surface hardnessafter forming. The hot-rolled steel sheet has a thickness of 3 to 20 mmand contains, in a chemical composition in mass percent (hereinafter thesame for chemical composition), C in a content of 0.3% or less(excluding 0%), Si in a content of 0.5% or less (excluding 0%), Mn in acontent of 0.2% to 1%, P in a content of 0.05% or less (excluding 0%), Sin a content of 0.05% or less (excluding 0%), Al in a content of 0.01%to 0.1%, and N in a content of 0.008% to 0.025%, with the remainderapproximately consisting of iron and inevitable impurities. In thehot-rolled steel sheet, the content of solute nitrogen is 0.007% ormore, and the contents of carbon and nitrogen meet a condition asspecified by the expression: 10C+N≦3.0. The microstructure of thehot-rolled steel sheet indudes pearlite in a content of less than 20% inarea percentage based on the total microstructure, with the remainderapproximately consisting of ferrite. The ferrite has an average grainsize of 3 to 35 μm.

In an embodiment, the hot-rolled steel sheet according to the aspect mayfurther contain, in the chemical composition, Cr in a content of 2% orless (excluding 0%) and/or Mo in a content of 2% or less (excluding 0%).

In another embodiment, the hot-rolled steel sheet according to theaspect may further contain, in the chemical composition, at least oneelement selected from the group consisting of Ti in a content of 0.2% orless (excluding 0%), Nb in a content of 0.2% or less (excluding 0%), andV in a content of 0.2% or less (excluding 0%).

In yet another embodiment, the hot-rolled steel sheet according to theaspect may further contain, in the chemical composition, B in a contentof 0.005% or less (excluding 0%).

In yet another embodiment, the hot-rolled steel sheet according to theaspect may further contain, in the chemical composition, at least oneelement selected from the group consisting of Cu in a content of 5% orless (excluding 0%), Ni in a content of 5% or less (excluding 0%), andCo in a content of 5% or less (excluding 0%).

In still another embodiment, the hot-rolled steel sheet according to theaspect may further contain, in the chemical composition, at least oneelement selected from the group consisting of Ca in a content of 0.05%or less (excluding 0%), at least one rare-earth element (REM) in a totalcontent of 0.05% or less (excluding 0%), Mg in a content of 0.02% orless (excluding 0%), Li in a content of 0.02% or less (excluding 0%), Pbin a content of 0.5% or less (excluding 0%), and Bi in a content of 0.5%or less (excluding 0%).

Advantageous Effects of Invention

According to the present invention, a microstructure mainly containingferrite having a predetermined average grain size is controlled so thatthe microstructure contains solute nitrogen in a certain amount, and thecarbon content and the nitrogen content meet a predetermined condition.This can provide a hot-rolled steel sheet that has lower deformationresistance during cold forming, thereby contributes to longer lives oftools, still resists fracture/cracking, and gives, after forming, a partsurely having predetermined surface hardness.

DESCRIPTION OF EMBODIMENTS

The hot-rolled steel sheet according to the present invention will beillustrated in detail below. The hot-rolled steel sheet according to thepresent invention is hereinafter also referred to as “steel sheetaccording to the present invention” or simply referred to as “steelsheet”. The steel sheet according to the present invention has acommonality with the hot-forged steel disclosed in PTL 2 in that solutenitrogen is contained in a certain amount and that the carbon contentand the nitrogen content are controlled so as to meet a predeterminedcondition. The steel sheet according to the present invention, however,differs from the hot-forged steel in that the upper limit of the carboncontent is relatively high, the steel sheet is controlled to include aferrite-pearlite dual-phase microstructure as the microstructure, andferrite grains are refined.

The steel sheet according to the present invention has a thickness of 3to 20 mm.

First of all, the steel sheet according to the present invention isdirected to one having a thickness of 3 to 20 mm. The steel sheet, ifhaving a thickness of less than 3 mm, may fail to surely have rigidityas a structure. In contrast, the steel sheet, if having a thicknessgreater than 20 mm, may hardly have the microstructure in the form asspecified in the present invention and hardly have desired effects. Thesteel sheet preferably has a thickness of 4 to 19 mm.

Next, the chemical composition constituting the steel sheet according tothe present invention will be described. All chemical elementshereinafter are indicated in mass percent.

Chemical Composition of Steel Sheet According to Present Invention

C in a Content of 0.3% or Less (Excluding 0%)

Carbon (C) significantly affects the formation of steel sheetmicrostructure, and the content thereof may be controlled so as to forma microstructure that is a ferrite-pearlite dual-phase microstructure,but mainly contains ferrite and contains pearlite in a minimized amount.The steel sheet, if containing carbon in excess, may have a higherpearlite fraction in the microstructure and might have excessively highdeformation resistance due to pearlite's work hardening. To preventthis, the carbon content in the steel sheet may be controlled to 0.3percent by mass or less, preferably 0.25% or less, more preferably 0.2%or less, and particularly preferably 0.15% or less. However, the steelsheet, if having an excessively low carbon content, may hardly undergodeoxidation during steel ingot making. To prevent this, the carboncontent may be controlled to preferably 0.0005% or more, more preferably0.0008% or more, and particularly preferably 0.001% or more.

Si in a Content of 0.5% or Less (Excluding 0%)

Silicon (Si) dissolves in the steel, causes the steel sheet to havehigher deformation resistance, and has to be minimized. To reducedeformation resistance, the Si content in the steel sheet may becontrolled to 0.5% or less, preferably 0.45% or less, more preferably0.4% or less, and particularly preferably 0.3% or less. However, thesteel sheet, if having an extremely low Si content, may hardly undergodeoxidation during steel ingot making. To prevent this, the Si contentmay be controlled to preferably 0.005% or more, more preferably 0.008%or more, and particularly preferably 0.01% or more.

Mn in a Content of 0.2% to 1%

Manganese (Mn) effectively deoxidizes and desulfurizes in the steelmaking process. Assuming that the steel has a higher nitrogen content,in this case, the steel sheet may be susceptible to fracture/cracking bydynamic strain aging hardening with the heat generation by mechanicalforming. Manganese, however, effectively contributes to betterformability in this process and restrains fracture/cracking. To havethese activities effectively, the Mn content in the steel sheet may becontrolled to 0.2% or more, preferably 0.22% or more, and morepreferably 0.25% or more. However, the steel sheet, if containing Mn inexcess, may have excessively high deformation resistance and may sufferfrom a heterogeneous microstructure due to segregation. To prevent this,the Mn content may be controlled to 1% or less, preferably 0.98% orless, and more preferably 0.95 percent by mass or less.

P in a Content of 0.05% or Less (Excluding 0%)

Phosphorus (P) is an impurity element and is inevitably contained in thesteel. Phosphorus, if contained in ferrite, segregates at ferrite grainboundaries to impair cold formability. This element also causes ferriteto undergo solute strengthening and cause the steel sheet to have higherdeformation resistance. To prevent this and to offer good coldformability, the phosphorus content is preferably minimized. However,extreme minimization of the phosphorus content may bring about anincrease in steel making cost. To prevent this in consideration ofprocess capability, the phosphorus content may be controlled to 0.05% orless, and preferably 0.03% or less.

S in a Content of 0.05% or Less (Excluding 0%)

Sulfur (S) is an inevitable impurity as with phosphorus, precipitates asa film of FeS at grain boundaries, and impairs formability. This elementalso causes hot brittleness. To prevent this and to provide betterdeformability, the sulfur content herein may be controlled to 0.05% orless, and preferably 0.03% or less. It is industrially difficult,however, to control the sulfur content to zero (0). However, sulfureffectively allows the steel sheet to have better machinability. Forbetter machinability, it is recommended for the steel sheet to containsulfur in a content of preferably 0.002% or more, and more preferably0.006% or more.

Al in a Content of 0.01% to 0.1%

Aluminum (Al) effectively contributes to deoxidation in the steel makingprocess. To have the deoxidation effect, the steel sheet may have an Alcontent of 0.01% or more, preferably 0.015% or more, and more preferably0.02% or more. However, the steel sheet, if having an excessively highAl content, may have lower toughness and be susceptible tofracture/cracking. To prevent this, the Al content may be controlled to0.1% or less, preferably 0.09% or less, and more preferably 0.08 percentby mass or less.

N in a Content of 0.008% to 0.025%

Nitrogen (N) causes static strain aging hardening after forming andthereby allows the steel sheet to have predetermined strength, thusbeing important. For this reason, the nitrogen content in the steelsheet may be controlled to 0.008% or more, and preferably 0.0085% ormore, and more preferably 0.009% or more. However, the steel sheet, ifhaving an excessively high nitrogen content, may be significantlyaffected not only by static strain aging hardening, but also by dynamicstrain aging hardening during forming to have higher deformationresistance, thus being unsuitable. To prevent this, the nitrogen contentmay be controlled to 0.025% or less, preferably 0.023 percent by mass orless, and more preferably 0.02% or less.

Solute Nitrogen in a Content of 0.007% or More

The steel sheet includes solute nitrogen in a predetermined amount. Thisaccelerates static strain aging hardening with less increase indeformation resistance. The amount of solute nitrogen is hereinafteralso referred to as “solute nitrogen content”. The steel sheet may havea solute nitrogen content of 0.007% or more so as to surely haverequired strength after cold forming. However, the steel sheet, ifhaving an excessively high solute nitrogen content, may have inferiorcold formability. To prevent this, the solute nitrogen content ispreferably controlled to 0.03% or less. As the total nitrogen content inthe steel sheet is 0.025% or less, the solute nitrogen content does notapproximately exceed 0.025%.

As used herein the term “solute nitrogen content” refers to the amountas determined by subtracting the total amount of nitrogen compounds fromthe total nitrogen content in the steel sheet. The determination isperformed based on Japanese Industrial Standard (JIS) G 1228. Practicalmethods for measuring the solute nitrogen content are exemplified asfollows:

(a) Inert Gas Fusion-Thermal Conductivity Analysis (Total NitrogenContent Measurement)

A specimen is cut out from a test sample, placed in a crucible, andfused in an inert gas stream to extract nitrogen. The extract istransferred to a thermal conductivity cell to measure a change inthermal conductivity to thereby determine the total nitrogen content.

(b) Ammonia Separation by Distillation-Indophenol Blue Absorptiometry(Total Nitrogen Compound Amount Measurement)

A specimen is cut out from the test sample, dissolved in a 10% AAelectrolytic solution, subjected to constant current electrolysis tomeasure the total amount of nitrogen compounds in the steel (steelsheet). The 10% AA electrolytic solution to be used is a non-aqueouselectrolytic solution that contains 10% of acetone and10% oftetramethylammonium chloride with the remainder being methanol and doesnot form a passive film on the steel surface.

About 0.5 g of the specimen sampled from the test sample is dissolved inthe 10% AA electrolytic solution, and formed undissolved residue(nitrogen compounds) is filtered through a polycarbonate filter having apore size of 0.1 μm. The undissolved residue is heated in sulfuric acid,potassium sulfate, and pure copper chips and is decomposed, and thedecomposed product is combined with the filtrate. The resulting mixture(solution) is treated with sodium hydroxide to be basic, subjected tosteam distillation, and distilled ammonia is absorbed by dilutedsulfuric acid. This is further combined with phenol, sodiumhypochlorite, and sodium pentacyanonitrosylferrate(III) to form a bluecomplex, and the absorbance of the blue complex is measured using anabsorptiometer to determine the total amount of nitrogen compounds.

The total amount of nitrogen compounds determined by the method (b) issubtracted from the total nitrogen content determined by the method (a)to give the solute nitrogen content.

The carbon and nitrogen contents meet the condition as specified by theexpression: 10C+N≦3.0

In the steel sheet according to the present invention, the solute carboncontributes to significantly better deformation resistance, but lesscontributes to static strain aging hardening. In contrast, the solutenitrogen less contributes to higher deformation resistance, but canaccelerate static strain aging hardening and can effectively contributeto higher hardness after forming. Based on this, the steel sheetaccording to the present invention essentially has the carbon contentand the nitrogen content meeting the condition as specified by theexpression: 10C+N≦3.0. The condition between the two elements ispreferably 0.009≦10C+N≦2.8, more preferably 0.01≦10C+N≦2.5, andparticularly preferably 0.01≦10C+N≦2.0. The condition is specified so asto increase the hardness after forming with less causing the increase ofdeformation resistance during forming. The steel sheet may have a carboncontent and a solute carbon content at certain levels, for the grainrefinement in the hot-rolled steel sheet and for the formability of thesteel sheet at certain level. However, the steel sheet, if having carbonand nitrogen contents not meeting the condition (if 10C+N is greaterthan 3.0), may have excessively high deformation resistance due toexcessive content(s) of carbon and/or nitrogen. In the inequality, thecoefficient of the carbon content is set 10 times the coefficient of thenitrogen content. This is set in consideration that the solute carbon,even contained in the same content with the solute nitrogen, increasesthe strength and deformation resistance of the hot-rolled steel sheetaccording to the present invention to a degree greater by an order ofmagnitude (10 times) as compared with the solute nitrogen.

The steel sheet according to the present invention basically containsthe chemical composition (elements) with the remainder approximatelyconsisting of iron and inevitable impurities. The steel sheet mayfurther contain any of following allowable elements within ranges notadversely affecting the operation of the present invention.

Cr in a Content of 2% or Less (Excluding 0%) and/or Mo in a Content of2% or Less (Excluding 0%)

Chromium (Cr) increases grain boundary strength to effectively allow thesteel to have better deformability. To have such activities effectively,the steel sheet preferably contain Cr in a content of 0.2%. However, thesteel sheet, if containing Cr in excess, may have higher deformationresistance and deteriorated cold formability. To prevent this, the Crcontent may be controlled to preferably 2% or less, more preferably 1.5%or less, and particularly preferably 1% or less.

Molybdenum (Mo) effectively allows the steel sheet to have higherhardness after forming and better deformability. To have such activitieseffectively, the steel sheet may contain Mo in a content of preferably0.04% or more, and more preferably 0.08% or more. However, the steelsheet, if containing Mo in excess, may have inferior cold formability.To prevent this, the Mo content may be controlled to preferably 2% orless, more preferably 1.5% or less, and particularly preferably 1% orless.

At Least One Element Selected from the Group Consisting of Ti in aContent of 0.2% or Less (Excluding 0%), Nb in a Content of 0.2% or Less(Excluding 0%), and V in a Content of 0.2% or Less (Excluding 0%)

These elements have a high affinity for nitrogen, are coexistent withnitrogen to form nitrogen compounds, and contribute to grain refinementof the steel. These elements also allow the formed product (steel sheet)after cold forming to have better toughness and better resistance tofracture/cracking. Each of the elements, however, fails to offer furtherbetter properties when contained in a content greater than the upperlimit. To prevent this, the contents of the elements may each becontrolled to preferably 0.2% or less, more preferably 0.001% to 0.15%,and particularly preferably 0.002% to 0.1%.

B in a Content of 0.005% or Less (Excluding 0%)

Boron (B) acts similarly to Ti, Nb, and V mentioned above. Specifically,boron has a high affinity for nitrogen, is coexistent with nitrogen toform nitrogen compounds, and contributes to grain refinement in thesteel. This element also allows the formed product (steel sheet) aftercold forming to have better toughness and better resistance tofracture/cracking. The steel sheet according to the present invention,when containing boron, can have a required solute nitrogen content andhave higher strength after cold forming. The boron content may thereforebe preferably 0.005% or less, more preferably 0.0001% to 0.0035%, andparticularly preferably 0.0002% to 0.002%.

At Least One Element Selected from the Group Consisting of Cu in aContent of 5% or Less (Excluding 0%), Ni in a Content of 5% or Less(Excluding 0%), and Co in a Content of 5% or Less (Excluding 0%)

These elements each effectively allow the steel to undergo strain aginghardening and to be hardened and effectively allow the steel sheet tohave higher strength after forming. To have such activities effectively,each of these elements may be contained in a content of preferably 0.1%or more, and more preferably 0.3% or more. However, these elements, ifcontained in excess, may have saturated effects of allowing the steel toundergo strain aging hardening and to be hardened and allowing the steelsheet to have higher strength after forming and may acceleratefracture/cracking. To prevent this, the contents of these elements mayeach be controlled to preferably 5% or less, more preferably 4% or less,and particularly preferably 3% or less.

At Least One Element Selected from the Group Consisting of Ca in aContent of 0.05% or Less (Excluding 0%), at Least One Rare-Earth Element(REM) in a Total Content of 0.05% or Less (Excluding 0%), Mg in aContent of 0.02% or Less (Excluding 0%), Li in a Content of 0.02% orLess (Excluding 0%), Pb in a Content of 0.5% or Less (Excluding 0%), andBi in a Content of 0.5% or Less (Excluding 0%)

Calcium (Ca) contributes to spheroiclization of MnS and other sulfideinclusions and allows the steel to have better deformability and bettermachinability. To have such activities effectively, Ca may be containedin a content of preferably 0.0005% or more, and more preferably 0.001%or more. However, Ca, if contained in excess, may have saturated effectsand is not expected to exhibit effects consistent with the content. Toprevent this, the Ca content may be controlled to preferably 0.05% orless, more preferably 0.03% or less, and particularly preferably 0.01%or less.

As with Ca, the rare-earth element(s) (REM) contributes tospheroidization of MnS and other sulfide inclusions and allows the steelto have better deformability and better machinability. To have suchactivities effectively, REM may be contained in a content of preferably0.0005% or more, and more preferably 0.001% or more. However, REM, ifcontained in excess, may have saturated effects and is not expected toexhibit effects consistent with the content. To prevent this, the REMcontent may be controlled to preferably 0.05% or less, more preferably0.03% or less, and particularly preferably 0.01 percent by mass or less.

As used herein the term “REM” refers to element or elements includinglanthanoid elements (fifteen elements from La to Lu), as well as Sc(scandium) and Y (yttrium). Of these elements, the steel sheetpreferably contains at least one element selected from the groupconsisting of La, Ce, and Y and more preferably contains La and/or Ce asREM.

Magnesium (Mg) contributes to spheroidization of MnS and other sulfideinclusions and allows the steel to have better deformability and bettermachinability, as with Ca. To have such activities effectively, Mg maybe contained in a content of preferably 0.0002% or more, and morepreferably 0.0005% or more. However, Mg, if contained in excess, mayhave saturated effects and is not expected to exhibit effects consistentwith the content. To prevent this, the Mg content may be controlled topreferably 0.02% or less, more preferably 0.015% or less, andparticularly preferably 0.01% or less.

Lithium (Li) contributes to spheroidimtion of MnS and other sulfideindusions and allows the steel to have better deformability, as with Ca.In addition, this element allows aluminum oxides to have lower meltingpoints and to be harmless and contributes to better machinability. Tohave such activities effectively, Li may be contained in a content ofpreferably 0.0002% or more, and more preferably 0.0005% or more.However, Li, if contained in excess, may have saturated effects and isnot expected to exhibit effects consistent with the content. To preventthis, the Li content may be controlled to preferably 0.02% or less, morepreferably 0.015% or less, and particularly preferably 0.01% or less.

Lead (Pb) effectively contributes to better machinability. To have suchactivities effectively, Pb may be contained in a content of preferably0.005% or more, and more preferably 0.01% or more. However, Pb, ifcontained in excess, may cause problems upon production, such asformation of roll marks. To prevent this, the Pb content may becontrolled to preferably 0.5% or less, more preferably 0.4% or less, andparticularly preferably 0.3 percent by mass or less.

Bismuth (Bi) effectively contributes to better machinability, as withPb. To have such activities effectively, Bi may be contained in acontent of preferably 0.005% or more, and more preferably 0.01% or more.However, Bi, if contained in excess, may have saturated effects forbetter machinability. To prevent this, the Bi content may be controlledto preferably 0.5 percent by mass or less, more preferably 0.4% or less,and particularly preferably 0.3% or less.

Next, the microstructure featuring the steel sheet according to thepresent invention will be described.

Microstructure of Steel Sheet According to Present Invention

As is described above, the steel sheet according to the presentinvention is based on a steel including a ferrite-pearlite dual-phasemicrostructure, in which the size of ferrite grains is controlled withina specific range.

Pearlite in Content of Less than 20%, with Remainder Being Ferrite

The steel sheet according to the present invention includes aferrite-pearlite dual-phase microstructure as its microstructure.Pearlite, if present in excess, may cause the steel sheet to haveinferior formability. To prevent this, the content of pearlite may becontrolled to less than 20%, more preferably 19% or less, furthermorepreferably 18% or less, and particularly preferably 15% or less in areapercentage. The remainder is approximately ferrite.

Ferrite Having Average Grain Size of from 3 to 35 μm

Ferrite grains constituting the ferrite phase may have an average grainsize of 3 to 35 μm so as to allow the steel sheet to have betterformability and satisfactory surface quality after forming. The steelsheet, if containing excessively fine (small) ferrite grains, may haveexcessively high deformation resistance. To prevent this, the averageferrite grain size may be controlled to 3 μm or more, preferably 4 μm ormore, and more preferably 5 μm or more. In contrast, the steel sheet, ifcontaining excessively coarse ferrite grains, may have inferior surfacequality after forming and may have inferior properties such as toughnessand fatigue properties. To prevent this, the average ferrite grain sizemay be controlled to 35 μm or less, preferably 30 μm or less, and morepreferably 25 μm or less.

Method for Measuring Area Percentages of Phases

The area percentages of the individual phases may be determined bysubjecting each test sample steel sheet to Nital etching, taking photosin five fields of view using a scanning electron microscope (SEM) at1000-fold magnification, and determining proportions of ferrite andpearlite by point counting.

Method for Measuring Average Grain Size

The average ferrite grain size may be measured typically in thefollowing manner. Specifically, sizes of ferrite grains present at threepoints, i.e., points corresponding to an outermost layer, one-fourth thethickness, and the central part in the thickness direction are measured.The grain size of one ferrite grain is measured in the following manner.The side surface in the rolling direction at each measurement point issubjected to Nital etching, photos in five fields of view in the portionare taken using a scanning electron microscope (SEM) at 1000-foldmagnification, and the diameter including the center of gravity of aferrite grain is determined by image analysis. The determined grainsizes are averaged to give an average ferrite grain size.

Next, a preferred method for producing the steel sheet according to thepresent invention will be illustrated below.

Preferred Method for Producing Steel Sheet According to PresentInvention

The steel sheet according to the present invention may be produced byany method, as long as a material steel having the chemical compositioncan be formed into a desired thickness. Typically, the steel sheet maybe produced by preparing a molten steel having the chemical compositionin a converter, subjecting this to ingot making or continuous casting togive a slab, and rolling the slab into a hot-rolled steel sheet having adesired thickness, under following conditions.

Molten Steel Preparation

The nitrogen content in the molten steel can be adjusted by adding a rawmaterial containing a nitrogen compound to the molten steel and/orcontrolling the atmosphere of the converter to be a nitrogen (N₂)atmosphere upon melting in the converter.

Heating

Heating before hot rolling is performed at a temperature of 1100° C. to1300° C. The heating is performed at such a high temperature in order todissolve nitrogen in an amount as much as possible while preventing theformation of nitrogen compounds. The lower limit of the heatingtemperature is preferably 1100° C., and more preferably 1150° C. Incontrast, heating to a temperature higher than 1300° C. is operationallydifficult.

Hot Rolling

Hot rolling is performed so that the finish rolling temperature be 880°C. or higher. The hot rolling, if performed at an excessively low finishrolling temperature, may cause ferrite transformation to occur at a hightemperature, may thereby cause carbide precipitates in ferrite tocoarsen, and may cause the steel sheet to have lower fatigue strength(inferior fatigue resistance). To prevent this, the hot rolling may beperformed at a finish rolling temperature at a certain level or higher.The hot rolling may be performed at a finish rolling temperature of morepreferably 900° C. or higher so as to allow austenite grains to coarsenand allow ferrite grains to have larger grain sizes to certain extent.The upper limit of the finish rolling temperature may be 1000° C.,because such a high finish rolling temperature as to exceed 1000° C. isdifficult to attain.

Hot Rolling Pass Schedule

The hot-rolled steel sheet according to the present invention has athickness of 3 to 20 mm. The refinement of ferrite grains to control theaverage ferrite grain size within the predetermined range requires notonly the control of the rolling temperature, but also the control oftandem rolling in the finish rolling to provide a final rollingreduction of 15% or more. In general, the finish rolling is performed asfive to seven passes of tandem rolling. In this process, a pass scheduleis determined from the viewpoint of control of holding fast with therollers and the steel sheet, and the final rolling reduction isgenerally set at about 12% to 13% or more, preferably 16% or more, andmore preferably 17% or more. With an increasing final rolling reduction(e.g., 20% to 30%), the hot rolling more effectively contributes to thegrain refinement. However, the upper limit of the final rollingreduction may be set at about 30% from the viewpoint of rolling control.

Rapid Cooling After Hot Rolling

Within 5 seconds after the completion of the finish rolling, theworkpiece is subjected to rapid cooling at a cooling rate (first rapidcooling rate) of 20° C./s or more, where the rapid cooling is stopped ata temperature (rapid cooling stop temperature) of from 580° C. to lowerthan 670° C. The rapid cooling is performed so as to obtain aferrite-pearlite dual-phase microstructure having predetermined phasefractions. The rapid cooling, if performed at a rate (rapid coolingrate) of less than 20° C./s, may accelerate pearlite transformation. Therapid cooling, if stopped at a temperature of lower than 580° C., mayaccelerate pearlite transformation or bainite transformation. The rapidcooling in both cases may hardly give a ferrite-pearlite steel havingpredetermined phase fractions and may cause the steel sheet to haveinferior bendability. In contrast, the rapid cooling, if stopped at atemperature of 670° C. or higher, may cause carbide precipitates inferrite to coarsen and may cause the steel sheet to have deterioratedfatigue strength. The rapid cooling may be stopped at a temperature ofpreferably 600° C. to 650° C., and more preferably 610° C. to 640° C.

Slow Cooling After Rapid Cooling Stop

After the rapid cooling stop, the workpiece is slowly cooled by naturalcooling or air cooling at a cooling rate (slow cooling rate) of 10° C./sor less for 5 to 20 seconds. This allows ferrite formation to proceedsufficiently and still contributes to appropriate refinement of carbideprecipitates in ferrite. The slow cooling, if performed at a coolingrate greater than 10° C./s and/or for a cooling time shorter than 5seconds, may cause insufficient formation of ferrite. In contrast, theslow cooling, if performed for a time longer than 20 seconds, may failto allow carbide precipitates to coarsen and may cause the steel sheetto have deteriorated fatigue strength.

Rapid Cooling and Coiling After Slow Cooling

After the slow cooling, the workpiece is subjected again to rapidcooling at a cooling rate (second rapid cooling rate) of 20° C./s ormore and coiled at a temperature of from higher than 300° C. to 450° C.This process is performed so as to allow the microstructure to mainlyinclude ferrite and to allow the steel sheet to have sufficientbendability at certain level. The second rapid cooling, if performed ata cooling rate (second rapid cooling rate) of less than 20° C./s, or thecoiling, if performed at a temperature of higher than 450° C., may causethe steel sheet to include an excessively large amount of pearlite. Incontrast, the coiling, if performed at a temperature lower than 300° C.,may cause the steel sheet to include martensite and retained austeniteand to have inferior bendability.

The present invention will be illustrated in further detail withreference to several examples (experimental examples) below. It shouldbe noted, however, that the examples are by no means intended to limitthe scope of the invention; that various changes and modifications cannaturally be made therein without deviating from the spirit and scope ofthe invention as described herein and that all such changes andmodifications should be considered to be within the scope of theinvention.

EXAMPLES

Steels having chemical compositions given in Table 1 below were made byvacuum melting, cast into ingots having a thickness of 120 mm, subjectedto hot rolling under conditions given in Table 2, and yielded hot-rolledsteel sheets. In each test (sample), rapid cooling after the completionof finish rolling down to the rapid cooling stop temperature wasperformed at a cooling rate of 20° C./s or more, and slow cooling afterthe rapid cooling stop was performed at a cooling rate of 10° C./s orless for 5 to 20 seconds.

The hot-rolled steel sheets obtained in the above manner were examinedto determine the solute nitrogen content, area percentages of individualphases, and average ferrite grain size in the microstructures of thesteel sheets by the measurement methods described as above inDescription of Embodiments.

As the formability of the hot-rolled steel sheets, 90-degree bendabilitywas evaluated by a 90-degree V-block test, because the steel sheets arethose having a thickness of about 10 mm. In the test, a test specimenwas pushed into a 90-degree die using a 90-degree punch, retrieved fromthe die, and the outside of the bent portion was visually observed. Thepunch has such a curvature that the ratio R/t of the punch insideminimum bend radius R to the steel sheet thickness t be 1. As a resultof the visual observation, a sample suffering from fracture wasevaluated as “×”; a sample not suffering from fracture, but sufferingfrom an apparent crack was evaluated as “Δ”; a sample suffering from nocrack although having fine asperities (wrinkles) was evaluated as “◯”;and a sample suffering from neither crack nor wrinkles was evaluated as“⊚”. As used herein the terms “fracture” and “crack” (cracking) referrespectively to one having a maximum gap distance of 1 mm or more andone having a maximum gap distance of less than 1 mm and aredistinguished from each other.

The hardness of the surface in the bent portion after the bend test wasmeasured to evaluate surface hardness after forming. The hardness wasmeasured as a Vickers hardness (Hv) of each test specimen after formingusing a Vickers hardness tester. The measurement was performed fivetimes with a load of 1000 g, at a measurement position of the centralpart corresponding to one-fourth the diameter (D) of the resulting part.

The results of the measurements are indicated in Table 3 below.

TABLE 1 Chemical composition (in mass percent) with the remainderconsisting of Fe and inevitable impurities Steel C Si Mn P S Al N 10C +N Others a 0.02 0.02 0.40 0.007 0.001 0.025 0.011 0.21 — b 0.05 0.020.40 0.007 0.001 0.022 0.008 0.51 — c 0.05 0.02 0.40 0.007 0.001 0.0220.023 0.52 — d 0.05 0.10 0.30 0.007 0.001 0.023 0.009 0.51 — e 0.05 0.400.20 0.007 0.001 0.024 0.009 0.51 — f 0.10 0.02 0.40 0.007 0.001 0.0220.010 1.01 — g 0.15 0.02 0.40 0.007 0.001 0.024 0.009 1.51 — h 0.20 0.020.40 0.007 0.001 0.022 0.010 2.01 — i 0.26 0.02 0.40 0.007 0.001 0.0230.009 2.61 — j 0.05 0.02 0.40 0.007 0.001 0.025 0.003 0.50 — k 0.05 0.020.40 0.007 0.001 0.025 0.030 0.53 — l 0.31 0.02 0.40 0.007 0.001 0.0250.008 3.11 — m 0.05 0.60 0.40 0.007 0.001 0.025 0.010 0.51 — n 0.05 0.020.15 0.007 0.001 0.025 0.012 0.51 — o 0.05 0.02 1.10 0.007 0.001 0.0250.011 0.51 — p 0.05 0.02 0.40 0.060 0.001 0.025 0.010 0.51 — q 0.05 0.020.40 0.007 0.060 0.025 0.011 0.51 — r 0.05 0.02 0.40 0.007 0.001 0.0050.012 0.51 — s 0.05 0.02 0.40 0.007 0.001 0.11  0.013 0.51 — t 0.05 0.020.40 0.007 0.001 0.025 0.010 0.51 Cr: 0.5, Mb: 0.03 u 0.05 0.02 0.400.007 0.001 0.025 0.010 0.51 Cu: 0.06, Ni: 0.15 v 0.05 0.02 0.40 0.0070.001 0.025 0.009 0.51 Ca: 0.0025, Li: 0.001 w 0.05 0.02 0.40 0.0070.001 0.025 0.009 0.51 Cr: 0.5, Mb: 0.03 x 0.30 0.02 0.40 0.007 0.0010.024 0.025 3.03 — (Element indicated with “—”: not added, underlineddata: out of the scope of the present invention)

TABLE 2 Hot rolling conditions Final Rapid cooling Thickness Heatingreduction Final rolling stop Coiling of hot-rolled Productiontemperature rate temperature temperature temperature sheet number Steel(° C.) (%) (° C.) (° C.) (° C.) (mm)  1 a 1250 16 920 620 430 10  2 a1250 18 911 607 405  4  3 a 1250 16 910 590 405 18  4* a  1000* 15  780* 545* 320 10  5* a 1250 15 900 600 410 25  6* a 1250  9* 893 593 414 10 7 b 1250 18 920 649 399 10  8 c 1250 16 922 607 311 10  9 d 1250 16 903593 311 10 10 e 1250 15 914 659 381 10 11 f 1250 16 923 594 395 10 12 g1250 16 886 596 369 10 13 h 1250 17 891 634 352 10 14 i 1250 18 901 615332 10 15 j 1250 16 894 617 346 10 16 k 1250 16 892 607 392 10 17 l 125017 911 594 331 10 18 m 1250 16 898 615 416 10 19 n 1250 17 913 612 32010 20 o 1250 15 922 623 427 10 21 p 1250 16 928 636 325 10 22 q 1250 16928 640 428 10 23 r 1250 17 910 616 374 10 24 s 1250 15 892 625 357 1025 t 1250 17 896 613 368 10 26 u 1250 15 896 627 315 10 27 v 1250 15 929624 391 10 28 w 1250 16 910 641 403 10 29 x 1250 17 907 605 411 10(Underlined data: out of the scope of the present invention, asteriskeddata: out of the recommended range)

TABLE 3 Microstructure Surface Solute Ferrite Pearlite hardness nitrogenarea area Average ferrite after Steel Production content percentagepercentage grain size 90-Degree forming No. Steel number (mass %) (%)(%) (μm) bendability (Hv) Remarks 1 a  1  0.0085 98 2 29 ⊚ 260 Inventivesteel sheet 2 a  2 0.008 94 6 16 ⊚ 270 Inventive steel sheet 3 a  30.008 98 2 33 ◯ 255 Inventive steel sheet 4 a  4* 0.003 96 4 30 ⊚ 180Comparative steel sheet 5 a  5* 0.009 88 12  45 X 190 Comparative steelsheet 6 a  6* 0.008 92 8 41 Δ 190 Comparative steel sheet 7 b  7 0.00797 3 24 ◯ 280 Inventive steel sheet 8 c  8 0.019 97 3 23 ◯ 305 Inventivesteel sheet 9 d  9  0.0085 97 3 21 ◯ 283 Inventive steel sheet 10 e 100.008 96 4 21 ◯ 290 Inventive steel sheet 11 f 11 0.009 95 5 19 ◯ 307Inventive steel sheet 12 g 12 0.008 90 10  13 ◯ 318 Inventive steelsheet 13 h 13 0.009 87 13  11 ◯ 332 Inventive steel sheet 14 i 14 0.00883 17   9 ◯ 351 Inventive steel sheet 15 j 15 0.002 96 4 25 ⊚ 171Comparative steel sheet 16 k 16 0.025 97 3 23 X — Comparative steelsheet 17 l 17 0.007 75 25   8 X — Comparative steel sheet 18 m 18 0.0085 96 4 24 X — Comparative steel sheet 19 n 19 0.010 97 3 23 ◯ 221Comparative steel sheet 20 o 20 0.009 95 5 21 X — Comparative steelsheet 21 p 21 0.009 96 4 26 X — Comparative steel sheet 22 q 22 0.010 973 27 X — Comparative steel sheet 23 r 23 0.011 97 3 26 X — Comparativesteel sheet 24 s 24 0.012 97 3 24 X — Comparative steel sheet 25 t 250.008 98 2 15 ◯ 290 Inventive steel sheet 26 u 26 0.009 97 3 16 ◯ 280Inventive steel sheet 27 v 27 0.008 97 3 17 ◯ 270 Inventive steel sheet28 w 28 0.008 98 2 15 ◯ 275 Inventive steel sheet 29 x 29 0.020 81 19 11 X — Comparative steel sheet (Underlined data: out of the scope of thepresent invention, asterisked data: out of the recommended range,Evaluation in 90-degree bendability: ⊚: very good, ◯: good, Δ: surfacecrack, X: fracture, —: Not measured due to fracture, Inventive steelsheet: one having very good or good 90-degree bendability and a surfacehardness after forming of 250 Hv or more, Comparative steel sheet: onenot meeting the conditions for the inventive steel sheet)

As is indicated in Table 3, Steel Sheet Nos. 1 to 3, 7 to 14, and 25 to28 employed steels having chemical compositions meeting the conditionsspecified in the present invention and were produced under recommendedhot rolling conditions. As a result, these gave inventive steel sheetshaving microstructures meeting the conditions specified in the presentinvention. The steel sheets had 90-degree bendability and surfacehardness after forming both meeting the acceptance criteria,demonstrating that the hot-rolled steel sheets have good coldformability during forming and still have predetermined surface hardness(strength) after forming.

In contrast, Steel Sheet Nos. 4 to 6, 15 to 24, and 29 are comparativesteel sheets not meeting at least one of the conditions for the chemicalcomposition and microstructure specified in the present invention. Thesesteel sheets did not meet at least one of the 90-degree bendability andsurface hardness after forming not meeting the acceptance criterion.

Typically, Steel Sheet No. 4 had a chemical composition meeting thecondition, but underwent heating before hot rolling at an excessivelylow temperature out of the recommended range, and included solutenitrogen in an insufficient content. The steel sheet had poor surfacehardness after forming.

Steel Sheet No. 5 had a chemical composition meeting the condition, buthad an excessively large thickness after hot rolling out of the specificrange. The steel sheet included coarsened ferrite grains and wasinferior both in 90-degree bendability and in surface hardness afterforming.

Steel Sheet No. 6 had a chemical composition meeting the condition, butunderwent hot rolling with an excessively low final rolling reductionout of the recommended range. The steel sheet included coarsened ferritegrains and was inferior both in 90-degree bendability and in surfacehardness after forming.

Steel Sheet No. 15 (Steel j) underwent hot rolling under conditionswithin the recommended range, but had an excessively low nitrogencontent, and thereby had poor surface hardness after forming.

In contrast, Steel Sheet No. 16 (Steel k) underwent hot rolling underconditions within the recommended range, but had an excessively highnitrogen content, and was inferior at least in 90-degree bendability.

Steel Sheet No. 17 (Steel l) underwent hot rolling under conditionswithin the recommended range, but had an excessively high carbon contentand failed to meet the condition as specified by the expression:10C+N≦3.0. The steel sheet included an excessively large amount ofpearlite and was inferior at least in 90-degree bendability.

Steel Sheet No. 18 (Steel m) underwent hot rolling under conditionswithin the recommended range, but had an excessively high Si content,and was inferior at least in 90-degree bendability.

Steel Sheet No. 19 (Steel n) underwent hot rolling under conditionswithin the recommended range, but had an excessively low Mn content, andwas inferior at least in surface hardness after forming.

In contrast, Steel Sheet No. 20 (Steel o) underwent hot rolling underconditions within the recommended range, but had an excessively high Mncontent, and was inferior at least in 90-degree bendability.

Steel Sheet No. 21 (Steel p) underwent hot rolling under conditionswithin the recommended range, but had an excessively high phosphoruscontent, and was inferior at least in 90-degree bendability.

Steel Sheet No. 22 (Steel q) underwent hot rolling under conditionswithin the recommended range, but had an excessively high sulfurcontent, and was inferior at least in 90-degree bendability.

Steel Sheet No. 23 (Steel r) underwent hot rolling under conditionswithin the recommended range, but had an excessively low Al content, andwas inferior at least in 90-degree bendability.

In contrast, Steel Sheet No. 24 (Steel s) underwent hot rolling underconditions within the recommended range, but had an excessively high Alcontent, and was inferior at least in 90-degree bendability.

In contrast, Steel Sheet No. 29 (Steel x) underwent hot rolling underconditions within the recommended range, but failed to meet thecondition as specified by the expression: 10C+N≦3.0, and was inferior atleast in 90-degree bendability.

These results demonstrate the applicability of the present invention.

While the present invention has been particularly described withreference to specific embodiments thereof it is obvious to those skilledin the art that various changes and modifications may be made withoutdeparting from the spirit and scope of the present invention.

The present application claims priority to Japanese Patent ApplicationNo. 2013-002640 filed on Jan. 10, 2013 and Japanese Patent ApplicationNo. 2013-056658 filed on Mar. 19, 2013, the entire contents of which areincorporated herein by reference.

INDUSTRIAL APPLICABILITY

The hot-rolled steel sheets according to the present invention aresuitable for automobile parts such as gears and other gearbox unitparts, and casings.

1. A hot-rolled steel sheet having excellent cold formability andsatisfactory surface hardness after forming, the hot-rolled steel sheethaving a thickness of 3 to 20 mm, the hot-rolled steel sheet comprising,in chemical composition in percent by mass (hereinafter the same forchemical composition): C in a content of 0.3% or less (excluding 0%); Siin a content of 0.5% or less (excluding 0%); Mn in a content of 0.2% to1%; Pin a content of 0.05% or less (excluding 0%); S in a content of0.05% or less (excluding 0%); Al in a content of 0.01% to 0.1%; N in acontent of 0.008% to 0.025%; with the remainder consisting of iron andinevitable impurities, a content of solute nitrogen being 0.007% ormore, and the contents of carbon (C) and nitrogen (N) meeting acondition as specified by expression: 10C+N≦3.0, a microstructure of thehot-rolled steel sheet comprising: pearlite in a content of less than20% in area percentage based on the total microstructure, with theremainder approximately consisting of ferrite, the ferrite having anaverage grain size of 3 to 35 μm.
 2. The hot-rolled steel sheetaccording to claim 1, further comprising, in the chemical composition,at least one element selected from the group consisting of: Cr in acontent of 2% or less (excluding 0%); Mo in a content of 2% or less(excluding 0%); Ti in a content of 0.2% or less (excluding 0%); Nb in acontent of 0.2% or less (excluding 0%) V in a content of 0.2% or less(excluding 0%) B in a content of 0.005% or less (excluding 0%); Cu in acontent of 5% or less (excluding 0%); Ni in a content of 5% or less(excluding 0%); Co in a content of 5% or less (excluding 0%); Ca in acontent of 0.05% or less (excluding 0%); at least one rare-earth element(REM) in a total content of 0.05% or less (excluding 0%); Mg in acontent of 0.02% or less (excluding 0%); Li in a content of 0.02% orless (excluding 0%); Pb in a content of 0.5% or less (excluding 0%); andBi in a content of 0.5% or less (excluding 0%).